1. Field of the Invention
This invention relates generally to a system for the control of valves in a subsea environment. More particularly, this invention relates to a system for actuating valves in oil and gas production wells and methods for controlling actuation of such valves.
2. The Prior Art
The control of subsea valves constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved as well as the risks associated with environmental and safety issues. As the exploration for oil and gas moves into ever deeper waters, the problems (and expense) associated with such activities gets ever larger.
Before describing the current state-of-the-art relative to such valve control systems and methods, a brief description will be made of the production system, per se, in need of control. Oil and gas that are produced in a well flows upward through the well tubing to a wellhead placed on the seabed. A so-called Christmas Tree is connected to the wellhead and contains a number of valves for regulating the produced fluids. These valves must be of a size to be able to contain the, often large, pressures in the well. In addition, a control module is usually provided, containing also a number of pilot valves, some for controlling the main Christmas Tree valves, but also valves for injection of chemical fluids into the well.
Known systems used for controlling the opening and closing of both the main gate valves in the Christmas Tree and others, for example injection valves, are either hydraulic, pneumatic or electro-hydraulic actuators, but the most common is the hydraulic type. These actuators generally utilize an externally controlled flow of hydraulic fluid, under pressure, to drive a piston within an actuator cylinder. This cylinder compresses a spring stack and moves the valve stem to open or close the valve depending on its configuration. In an emergency situation when the valve must be closed quickly, the spring force is used to achieve this. In a subsea environment, it is mandatory to provide the actuators with such fail-safe close configuration.
The actuators may be driven directly, that is being supplied with fluid through a line extending to the surface, or from an accumulator located on the seabed at or near the well. In the latter case, pilot valves are arranged to control the supply of fluid to the actuators. Pilot valves can be controlled hydraulically, that is with a secondary supply line from the surface, or more often electrically and is therefore called an electro-hydraulic system.
However, hydraulic actuators have a number of operational disadvantages that would make the actuator unusable for some subsea applications. For example, as the distance between the hydraulic actuator and the topside control center increases, the amount of hydraulic fluid needed to fill the lines and operate the valves also increases. Over long distances the fluid can lose pressure and therefore accumulators have to be installed close to the actuator to back up the pressure. Within some existing systems the time between a signal being sent from a control center and the actuators responding to the signal can be a matter of minutes, which in emergency situations can be too long. Furthermore, when the distance between the two sites reaches an optimum length, the hydraulic actuator will not be able to operate at all. An increase in water depth can also affect the actuator performance. The cost and size of the hydraulic lines and umbilicals also greatly increase as the operational depths and distances compound. It has therefore become apparent that a new design of actuator control is required that will not be affected by depth/distance and that will have a fast response time, and it has been decided that an all-electric valve actuator will meet these requirements.
U.S. Pat. No. 5,497,672 to Appleford et al describe a valve actuator comprising a stem extending through the actuator for coupling to the valve such that axial movement of the stem opens and closes the valve, an outer shaft arranged coaxially around the stem and in threaded engagement therewith and means coupling the shaft to an electric motor to cause rotational and thus axial movement of the shaft relative to the stem against a spring bias. The shaft is releasable retained in a predetermined position when the valve is open with the spring held in compressed state by a solenoid-actuated latching mechanism. De-activation of the solenoid mechanism, for example by a loss of electric power, releases the shaft to allow axial movement of both the shaft and stem, under action of the spring, in order to close the valve.
The above mentioned actuator needs the spring for providing the required fail safe characteristics. It is therefore not a true all electric valve actuator. Since the valves are designed so that the actuators must overcome the pressure differential between the pressure in the well and the ambient (hydrostatic) pressure, in order to open or close the valve, this spring is considered necessary to be able to close the valve at the loss of electric power. If the well pressure is higher than the ambient pressure, the return spring must be designed large enough to be able to keep the valve shut. Contraversive, if the ambient pressure is larger than the well pressure, the driving fluid pressure must be larger. At greater depths, the hydrostatic pressure will normally be several orders of magnitude higher than the well pressure and the driving fluid must not only be able to move the actuator against the return spring but also overcome the pressure caused by the water depth.
When actuators of this type has found little application for subsea use it is because of the many disadvantages found. The size of the return spring, which as mentioned above is dependent upon well pressure and ambient pressure, results in that the actuator being of the same magnitude of size as in a conventional hydraulic actuator and neither weight nor space is reduced. The latch mechanism is less operational safe and even short-time power losses will make the actuator close the valve.
U.S. Pat. No. 5,519,295 to Jatnieks describes an electric actuator comprising a stem extending through the actuator for coupling to a component such as a valve such that axial movement of the stem opens and closes the valve, an outer shaft in threaded engagement therewith via a gear train and means coupling the shaft to an electric motor to cause rotational and thus axial movement of the shaft relative to the stem. The actuator has a controller having input power connected across a capacitor and control terminals for controlling the power applied from the input power terminals to the motor which comprises the actuator""s prime mover. Electric energy is stored in the capacitor and a sensor monitors the presence of power provided to the controller""s input power terminals When failure of this power is sensed, the capacitor current flows to the controller, which drives the actuator to toward a the preselected (normally closing) position.
This design, however, is not very useful in a subsea environment, since a capacitor is not reliable in seawater and would need costly shielding to avoid short-circuiting, which means that it must be housed in a one atmosphere container.
EPO patent application no. 984,133 to Cooper Cameron Corp. describes a valve actuating module in a subsea environment having a plurality of valve actuators for operating respective valve elements. The module is arranged to receive primary power and control signals from an external source in order to operate the valves. The necessary failsafe condition is achieved by providing the motor with a local back-up power source which, in the event of loss of primary power, is sufficient to drive the motor to return the valve to a safe position, the back-up power source being a battery which is kept charged by the primary power source through a umbilical.
Note may also be made to the article  less than  less than Subsea Electric Valve Actuator greater than  greater than , by Sigbjrn Sangesland and Michael Golan and published in  less than  less than Proceedings from the 8th Underwater Technology Conference held in Bergen, Norway Mar. 14-16, 1994. This comprises an overall description of the then current thinking about using electrically driven valve actuators and discussions of the pros and cons of such a system, including suggesting using a battery as back-up.
Again, the big disadvantage of all these designs is that even a short-time power loss will make the actuator to close the valve. This is not desirable, since loss of power can have many causes, some unrelated to conditions in the well. It may well be that such non-important power losses can occur so frequently that the batteries are not sufficiently charged before the next failure happens. An uncontrolled closing of the valve may also cause disruptions to the system even after the valve is re-opened, for example because of hydrate formation.
GB Application no. 2,216,570 to Baker Hughes Inc. describes a downhole safety valve actuator that comprises an axially shiftable valve head which is movable between open and close positions by an actuating member manipulated by an electric motor. The electric motor is powered by a battery and the energization and de-energization of the electric motor is controlled by electromagnetic waves. A locking mechanism, consisting of a biased return spring controlled by an electric solenoid is also powered by the battery. A surface mounted electromagnetic wave transmitter transmits an electromagnetic signal downwardly through the earth and is received by an antenna mounted in the well and supplied to an electronic receiver unit-The electronic unit receives and amplifies the received signal and supplies it to a central processor. The processor in turns controls the supply of energy from the batteries to the electric motor for shifting the valve between open and closed positions.
The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the control system of the present invention. In accordance with a first embodiment of the present invention, a valve control system for a subsea valve is provided that will automatically or semi-automatically operate the valve in response to sensed production or environmental parameters. An important feature of this invention is thus that the actuator will not immediately close the valve as a response to power loss, but will check other system parameters before making this decision.
In accordance with the present invention, therefore, an actuator system for actuating a valve in a subsea environment comprises a spindle axially movable between at least first and second operating positions, whereby, moving the actuator causes the valve element to move between operating positions, at least one electric motor with a rotatable output shaft, a transmission for transmitting the rotational output of the shaft into axial movement of the spindle, at least one electrical storage unit providing a source of primary power disposed proximate to the motor, and at least one control unit connected to each of said electrical storage unit, said motor and an external electrical source disposed remotely from said motor, where the control unit comprises first switching means for selectively transmitting electrical power from said electrical storage unit to said motor, second switching means for selectively transmitting electrical power from said external electrical power source to either said electrical storage unit or said motor, first sensing means for sensing the state of said electrical storage unit, said first sensing means generating a first sensor signal, second sensing means for sensing the state of said external electrical power source, said second sensing means generating a second sensor signal, and programmable processor means, said processor means being programmed to determine the desired operating position of said valve spindle and the desired source of electrical power for said motor using a plurality of input signals including at least said first and second sensor signals, and send appropriate control signals to said first and second switching means to selectively start or stop the transmission of electrical power from said desired source of electrical power to said motor to achieve said desired operating position of said valve spindle.
With this arrangement, the coupling, which only transmits the rotary motion from the motor into axial motion of the spindle, is unaffected by hydrostatic pressure and a return spring is nor required. Thus the electric power to operate the device is much reduced as all that is required is sufficient power to overcome the friction between the device element and housing seals. Further, because each actuator is provided with its own self-contained power source, the transmission power loss can be significantly reduced.
In a preferred embodiment, the motor, and the power source is housed in a module which is arranged to be coupled to, and decoupled from, the wellhead assembly by, for example, a diver or ROV. In this case, the coupling between the rotatable actuator and the rotatable part of he device will be a torque-transmitting interface, such as splined or keyed coupling, which can be engaged and disengaged by relative axial movement. Preferable this coupling is of a standardized designed, that will interface with a ROV manual override tool, to ensure operation of the valve even with the module disengaged from the device.
It is advantageous to use two motors coupled to the same output shaft, with a battery each. This ensures a redundancy and a safety factor that results in safer operation.
By using a battery as the principal power source, the large and bulky primary power umbilical can be significantly reduced, resulting in lower costs.
The control unit should be an intelligent programmable processor that can make decisions based on the state of the system, ensuring that the valves do not close for minor glitches in the system.
In accordance with a second embodiment of the invention there is disclosed a method for controlling actuation of a valve installed in a subsea environment, said valve comprising a spindle axially movable between at least first and second operating positions, said method comprising the steps of providing an electrically powered actuator for moving said valve spindle between said operating positions; providing an electrical storage unit disposed proximate to said motor; providing an external electrical power source located remotely from said motor; monitoring the state of said electrical storage unit; monitoring the state of said external power source; determining the desired operating position of said valve spindle and the desired source of power for said motor based on states of said electrical storage unit and said external power source; and connecting said desired power source to said motor such that said valve spindle is moved to said desired operating position.